The weight of the concrete and the steel superstructure in a bridge, and thus the resultant dead load in each of the critical members, are well known when the bridge is initially constructed, assuming all goes to plan. However, major maintenance, repair, or any significant damage to the structure can cause the loads to redistribute and thus change the dead loads and the load path. Therefore, it is important to know what these new dead loads and load paths are to ensure safe and reliable operation of the bridge. The in-service dead loads in metal bridge components can be measured using Proto’s x-ray diffraction (XRD) systems quickly and cost effectively without disrupting structure use.
The Manhattan approach to the Brooklyn Bridge was opened in 1883. Known as the Franklin Square Bridge, the span is made up of six parallel wrought iron eyebar trusses. When the bridge was built, the deck consisted of a two lane carriageway on each side of the bridge above the outer two trusses. Since then, the transit tracks have been removed and replaced by concrete decks, increasing the dead load of the bridge by 55 percent. This increased load caused concern that some members of the eyebars may be experiencing significantly high loads.
Additionally, there was concern that many of the pins connecting the eyebars may have deformed, resulting in additional stress redistributions around the pins and eyebars. Using a Proto iXRD portable x-ray diffraction system, Proto technicians successfully measured the dead loads in the eyebars and found that not all of the eyebars bore the same load; some of the eyebars bore very high loads approaching yield, while others bore very small loads approaching zero. This information assisted in the determination that the bridge was unsafe. As a result, six steel arches were added to support the existing trusses.
